Conventional automotive internal combustion engines operate with one or more camshafts controlling the timing and lift of the intake and exhaust valves, according to a predetermined lift schedule. With this type of mechanical structure, the lift schedule is fixed. However, under different engine operating conditions, the optimum lift schedule varies. Thus, the lift schedule must be a compromise of the optimum lift schedule needed for the different operating conditions. To accommodate full throttle engine operation, which requires significant air intake, an aggressive lift schedule must be used. At part load operating conditions, however, the intake air must then be throttled to prevent too much air from entering the cylinder. Consequently, this causes parasitic losses due to the throttling.
It is desirable to eliminate throttling losses by eliminating the need for an air throttle, without losing the effective compression ratio. One possible way to accomplish this is to close the intake valve before piston bottom dead center (BDC) during the intake stroke. However, the gas in the cylinder will then experience expansion during the end of the intake stroke with resultant cooling. Cooling of the gas can be detrimental to engine performance. Therefore, the need arises for a way to maintain the proper temperature of the gas at combustion when it undergoes a cooling due to expansion during the end of the intake stroke. This would improve combustion characteristics and provide better fuel economy.
Further, in internal combustion engines used in vehicles today, some of the exhaust gas is recirculated, by an external exhaust gas recirculation (EGR) system, to control the nitrogen oxide formation and to retain the maximum unburned hydrocarbons in the cylinder and allow for hotter intake gas for better evaporation of fuel. It is desired to eliminate the need for an external EGR system to reduce the cost and complexity that yields increased maintenance requirements. Additionally, for environmental reasons, it is desired to maintain as much of the unburned hydrocarbons in the cylinder as possible rather than allowing them to flow out in the exhaust.
It is understood that the distribution of unburned hydrocarbons in the cylinder charge at the beginning of the exhaust stroke is uneven. A substantial part of the unburned hydrocarbons that come out of the piston ring crevices at the end of the expansion stroke remain concentrated in the bottom part of the cylinder near the piston. If this part of the cylinder charge can be prevented from being discharged into the exhaust port, a substantial reduction in hydrocarbon emissions can be achieved. Thus, in order to maintain the greatest amount of unburned hydrocarbons within the cylinder, it is desired that the part of the exhaust charge with the highest concentration of unburned hydrocarbons be prevented from flowing out through the exhaust port. Furthermore, if some hot gas can temporarily reside in the intake port, it will increase the intake air temperature, which promotes better evaporation of fuel injected into the port, especially during engine cold start and warm-up. This, too, improves hydrocarbon emissions.
The enhancement of engine performance attainable by varying the acceleration, velocity and travel time of the intake and exhaust valves in an engine is well known and appreciated in the art. The increasing use and reliance on microprocessor control systems for automotive vehicles and increasing confidence in hydraulic and electric as opposed to mechanical systems is now making substantial progress possible. The almost limitless flexibility with which the intake and exhaust events (timing strategy) can be varied in an engine with a camless valvetrain can lead to substantial improvements in engine operation.
However, none of the present systems and methods of operation provide a variable engine valve control system that substitutes for the external EGR system in today's engines to reduce harmful emissions by returning a portion of unburned hydrocarbons back to the cylinder while at the same time promoting better evaporation of fuel, while also eliminating air throttling losses without reducing the effective compression ratio and while avoiding problems caused by low air temperature resulting from early intake valve closure. The present system optimizes engine performance, especially at part load engine operation.